The present invention relates to a color picture tube, and more particularly to a color picture tube having a flat external surface, the least brightness difference and an excellent contrast.
For providing the enhancement of the luminosity factor of a screen, the prevention of the glare due to the reflection of external light and the like, a color picture tube with a flat panel surface has been developed. From a viewpoint of assuring the mechanical strength against the atmospheric pressure while maintaining the flat external surface, it is necessary to make the thickness of the glass at the peripheral portion of a panel thicker than the thickness of the glass at the central portion of the panel. Furthermore, in case a shadow mask which is shaped by a press is adopted, a curvature is necessary for assuring the mechanical strength of the curved surface of the shadow mask. For this end, it is necessary to provide a curvature to the inside of the panel. From this aspect, the thickness of the peripheral portion of the panel becomes greater than the thickness of the central portion. However, since the panel glass absorbs light, the difference of thickness of the glass appears as the difference of the brightness between the peripheral portion and the central portion and hence, the image quality of the displayed image is deteriorated.
To compensate for this drawback, there have been following prior arts. U.S. Pat. No. 5,660,876, Japanese Laid-Open Publication Hei 5-299034/1993 and Japanese Laid-Open Publication Hei 5-182602/1993 disclose a constitution where a visible light absorption layer is formed of a black dye and the density of the black dye is higher at the central portion than the peripheral portion of a panel thus making the brightness uniform. In this method, the light absorption for enhancing the contrast of a displayed image and the correction of the grading of the brightness are both performed by a black dye layer. In case the light absorption is increased by the single film in such a manner, however, it is inevitable to increase the light absorption coefficient of the film and hence, the light from a phosphor is reflected by the absorption layer (back surface reflection) so that the deterioration of focusing and a ghost phenomenon occur thus degrading the displayed image.
Japanese Laid-Open Publication Hei 11-283531/1999 discloses a constitution where a colored film having a varied density is stuck to the surface of a panel such that the difference of transmittance of the panel glass can be compensated for. This has a problem that color films having various transmittance grading must be prepared for respective color cathode ray tubes.
Japanese Laid-Open Publication Hei 11-307016/1999 further discloses a constitution where a functional film which changes the light transmittance so as to compensate for the difference of transmittance of a panel glass is stuck to the surface of the panel. This also has a problem that the functional films having various transmittance grading must be prepared for respective color cathode ray tubes.
Japanese Laid-Open Publication Hei 6-139964/1994 discloses a film which has functions composed of the anti-reflection, the anti-static and the enhancement of contrast. A neutral filter or a color selective filter is formed in this film for enhancing the contrast. The publication, however, fails to disclose the countermeasures for the difference of brightness between the central portion and the peripheral portion, that is, the grading of brightness. Japanese Laid-Open Publication Hei 11-143371/1999 discloses a film whose adhesive layer has a light absorbing function. This known example also fails to disclose the countermeasures for the grading of brightness.
A color cathode ray tube of the present invention forms a coating layer which corrects the grading of brightness on the surface of a panel glass. Since the coating layer of the present invention aims at the correction of the grading brightness, the light absorption at the peripheral portion of a panel is minimized. Furthermore, a film is stuck on this coating layer so as to cover the coating layer. A light absorbing function is provided by this film so as to enhance the contrast of a displayed image. The film which covers the coating layer may preferably be a film which is capable of absorbing an approximately constant amount of light over the entire surface of the panel. In accordance with the present invention, the light absorption coefficient of visible light of the coating layer formed on the external surface of the panel is set to be small, and so the back surface reflection by the coating layer can be minimized. On the other hand, with the provision of the film, a necessary amount of light absorption can be obtained, and, hence, a necessary contrast can be easily obtained as well.